1. Field of the invention
The present invention relates to an oil pressure control system for an automatic transmission system for use with automobiles and, more particularly, to an improvement in an oil supply system for a buffering accumulator incorporated in the oil pressure control system for reducing shock caused in speed shifting.
2. Description of the Prior Art
In an automatic transmission for use with automobiles which includes a fluid torque converter, a transmission gear and friction engaging means for establishing a selected transmission engagement in said transmission gear and is adapted to be controlled by an oil pressure control system, an accumulator is generally provided in an oil pressure circuit for operating said friction engaging means in order to accomplish a smooth changing over of the automatic transmission system by reducing the shock which is caused by the engagement of the friction engaging means. An accumulator for the abovementioned purpose conventionally comprises a fluid displacing element such a cylinder-piston means wherein the piston is flexibly supported by a spring means which counteracts the oil pressure existing in the oil pressure circuit for operating the friction engaging means. Furthermore, it is known to supply an oil pressure generated in the oil pressure control system to said cylinder-piston means as a back pressure for the piston in order to control the stiffness of the buffering accumulator. However, since a particular friction engaging means such as the rear clutch is engaged in a plurality of operating conditions, i.e. D Range the highest speed (usually 3rd speed) condition as well as R Range, the back pressure supplied to the cylinder-piston type buffering accumulator which is adjusted to provide favourable stiffness in an operating condition will not be proper for the other operating condition. Generally, a higher stiffness of the buffering accumulator is favourable for R Range when compared with D Range the highest speed shifting condition. Conventionally, the line pressure generated in the oil pressure control system has been mostly employed as the back pressure for the accumulator. Since the line pressure is usually raised or lowered according to increase or decrease of the torque to be transmitted by the transmission system, it becomes higher in R Range than in D Range the highest speed shifting condition. Therefore, this performance has generally conformed to the requirement with regard to the stiffness of the buffering accumulator. However, for the modern oil pressure control system, it is required that the stiffness of the buffering accumulator is more delicately controlled as precisely adapted to the throttle opening so that the shock in the shifting be further reduced. To meet with this requirement, it has been proposed to generate a particular oil pressure by a valve means for use as the back pressure for the accumulator instead of the conventional line pressure. However, such a system is still bound with the drawback that the back pressure is not adjusted for the change of the shifting condition like the change between R Range and D Range 3rd speed, thereby making it difficult to obtain favourable shock reducing performance in all shifting ranges.